Structural and regulatory gene mutations affecting carbohydrate and pyrimidine metabolism will be isolated and analyzed in Chinese hamster and human cell lines in tissue culture. The genetic rather than epigenetic origin of these markers will be ascertained through studies of conditional lethal mutations at specific loci, interallelic complementation, and physiochemical properties of enzymes. Continued efforts will be made to elucidate the molecular basis of 67 galactose-negative pleiotropic mutants we have isolated in the hamster cells. Experiments are designed to search for constitutive and promotor mutations at the X-linked locus controlling glucose-6-phosphate dehydrogenase. In addition, repression and introduction of enzyme activities by substrates and regulatory gene mutations leading to an over-production of enzymes in the pyrimidine biosynthetic pathway will be investigated. It is hoped that these studies will furnish a firmer basis to the study of the origin, expression and rates of mutation in somatic mammalian cells and extrapolation from studies of this nature to the intact organism.